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Home / DTC / U0021 — Low Speed CAN Communication Bus (+) Open

U0021 — Low Speed CAN Communication Bus (+) Open

Detailed page for trouble code U0021.

34,354codes
59brands
11,863generic
22,491specific
All codes P-codes (Powertrain) B-codes (Body) C-codes (Chassis) U-codes (Network/User) Tesla Alerts
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Code

U0021

Generic U — Network/User

Low Speed CAN Communication Bus (+) Open

Brand: Generic
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Broken or disconnected CAN + conductor (open circuit)
  • Corroded or loose connector/pin at a module or splice
  • Short/open at a splice or wiring harness damage (chafing, rodent damage)
  • Failed CAN transceiver or control module
  • Missing or damaged termination resistor(s)
  • Water intrusion or contamination in connectors

Symptoms

  • Communication-related DTCs (U-codes) and MIL illumination
  • Loss or intermittent operation of modules on the LS-CAN network (e.g., body, door, mirror, HVAC, instrument cluster functions)
  • No or garbled data from one or more modules when scanned
  • Systems that rely on the LS-CAN remain inoperative or default to backup mode

What to check

  • Use a scan tool to confirm active U0021 and any additional communication codes; record freeze-frame and module list
  • Visually inspect LS-CAN wiring, connectors, splices and modules for damage, corrosion, or water entry
  • Check module power and ground circuits feeding the CAN transceivers (fuses, battery feed, chassis grounds)
  • Measure termination resistance between CAN+ and CAN− with ignition off (expected ≈ 60 ohms for a properly terminated two-terminator bus)
  • Perform continuity check of CAN+ conductor between modules and to splice points (with harness connectors disconnected where practical)
  • Use an oscilloscope or CAN bus monitor to observe CAN+ and CAN− waveforms while cranking/ignition on

Signal parameters

  • Clean, healthy CAN bus: recessive state — both lines near mid-supply (~2.5 V); dominant state — CAN_H rises (~3.5 V) and CAN_L falls (~1.5 V), producing ~2 V differential
  • Termination resistance (two 120 Ω in parallel) measured across CAN+ and CAN− with key off: ≈ 60 Ω
  • Open on CAN+ typically shows no toggling on CAN+ line and an idle voltage pulled toward module input or open-circuit voltage
  • Oscilloscope shows valid CAN frames (differential pulses) when network is healthy; an open conductor will remove expected differential transitions

Diagnostic algorithm

  1. Retrieve and record all active/pending codes and freeze-frame data. Note which modules are not responding on the scanner.
  2. Perform a visual inspection of LS-CAN harness routing, connectors, and modules for obvious damage or corrosion. Repair any obvious faults.
  3. Verify power and grounds to modules that interface to the LS-CAN (check fuses, fused links and chassis grounds). Repair as needed.
  4. With ignition off, measure resistance between CAN+ and CAN− at an accessible connector to verify termination (~60 Ω).
  5. Check continuity of the CAN+ conductor from the failing module connector back through splices to other network nodes. Repair any opens or high-resistance connections.
  6. With ignition on, use a multimeter/oscilloscope to monitor CAN+ and CAN− signals while comparing to a known-good vehicle or known-good segment. Look for absence of toggling on CAN+ (open) or abnormal voltages.
  7. Isolate the fault by disconnecting modules one at a time (where safe and practical) to see when the bus behavior or codes change—this can identify the segment or module causing the open.
  8. If a module is suspected, inspect and repair the connector and wiring; if harness and connectors are good and the bus is still open, replace or repair the defective module or its CAN transceiver per manufacturer instructions.
  9. After repairs, clear codes and verify network communications and affected systems return to normal during a test drive.
  10. Safety note: take usual precautions when probing live circuits. If performing soldering or splicing, disconnect battery where required by vehicle manufacturer procedures.

Likely causes

  • Damaged wire or pin at a harness connector where the LS-CAN branch connects
  • Corroded connector or poor pin-to-socket contact at a network module
  • Open splice or broken conductor inside loom
  • Failed module transceiver on one end of the LS-CAN segment
  • Removed or burnt termination resistor causing loss of network reference

Fault status

⚠️ Status
Low Speed CAN bus positive conductor open — communication lost or intermittent on LS-CAN network. Inspect wiring, connectors, terminations, and module transceivers.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

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Code

U0021

HYUNDAI U — Network/User

Low Speed CAN Communication Bus (+) Open

Brand: HYUNDAI
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Broken or disconnected CAN + conductor (open circuit)
  • Corroded or loose connector/pin at a module or splice
  • Short/open at a splice or wiring harness damage (chafing, rodent damage)
  • Failed CAN transceiver or control module
  • Missing or damaged termination resistor(s)
  • Water intrusion or contamination in connectors

Symptoms

  • Communication-related DTCs (U-codes) and MIL illumination
  • Loss or intermittent operation of modules on the LS-CAN network (e.g., body, door, mirror, HVAC, instrument cluster functions)
  • No or garbled data from one or more modules when scanned
  • Systems that rely on the LS-CAN remain inoperative or default to backup mode

What to check

  • Use a scan tool to confirm active U0021 and any additional communication codes; record freeze-frame and module list
  • Visually inspect LS-CAN wiring, connectors, splices and modules for damage, corrosion, or water entry
  • Check module power and ground circuits feeding the CAN transceivers (fuses, battery feed, chassis grounds)
  • Measure termination resistance between CAN+ and CAN− with ignition off (expected ≈ 60 ohms for a properly terminated two-terminator bus)
  • Perform continuity check of CAN+ conductor between modules and to splice points (with harness connectors disconnected where practical)
  • Use an oscilloscope or CAN bus monitor to observe CAN+ and CAN− waveforms while cranking/ignition on

Signal parameters

  • Clean, healthy CAN bus: recessive state — both lines near mid-supply (~2.5 V); dominant state — CAN_H rises (~3.5 V) and CAN_L falls (~1.5 V), producing ~2 V differential
  • Termination resistance (two 120 Ω in parallel) measured across CAN+ and CAN− with key off: ≈ 60 Ω
  • Open on CAN+ typically shows no toggling on CAN+ line and an idle voltage pulled toward module input or open-circuit voltage
  • Oscilloscope shows valid CAN frames (differential pulses) when network is healthy; an open conductor will remove expected differential transitions

Diagnostic algorithm

  1. Retrieve and record all active/pending codes and freeze-frame data. Note which modules are not responding on the scanner.
  2. Perform a visual inspection of LS-CAN harness routing, connectors, and modules for obvious damage or corrosion. Repair any obvious faults.
  3. Verify power and grounds to modules that interface to the LS-CAN (check fuses, fused links and chassis grounds). Repair as needed.
  4. With ignition off, measure resistance between CAN+ and CAN− at an accessible connector to verify termination (~60 Ω).
  5. Check continuity of the CAN+ conductor from the failing module connector back through splices to other network nodes. Repair any opens or high-resistance connections.
  6. With ignition on, use a multimeter/oscilloscope to monitor CAN+ and CAN− signals while comparing to a known-good vehicle or known-good segment. Look for absence of toggling on CAN+ (open) or abnormal voltages.
  7. Isolate the fault by disconnecting modules one at a time (where safe and practical) to see when the bus behavior or codes change—this can identify the segment or module causing the open.
  8. If a module is suspected, inspect and repair the connector and wiring; if harness and connectors are good and the bus is still open, replace or repair the defective module or its CAN transceiver per manufacturer instructions.
  9. After repairs, clear codes and verify network communications and affected systems return to normal during a test drive.
  10. Safety note: take usual precautions when probing live circuits. If performing soldering or splicing, disconnect battery where required by vehicle manufacturer procedures.

Likely causes

  • Damaged wire or pin at a harness connector where the LS-CAN branch connects
  • Corroded connector or poor pin-to-socket contact at a network module
  • Open splice or broken conductor inside loom
  • Failed module transceiver on one end of the LS-CAN segment
  • Removed or burnt termination resistor causing loss of network reference

Fault status

⚠️ Status
Low Speed CAN bus positive conductor open — communication lost or intermittent on LS-CAN network. Inspect wiring, connectors, terminations, and module transceivers.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

Similar codes

U0100 U0073 U0155 U0121 U0115 U0105
371

Browse 371 HYUNDAI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

HYUNDAI

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+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
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Code

U0021

MERCEDES-BENZ U — Network/User

Low Speed CAN Communication Bus (+) open

Brand: MERCEDES-BENZ
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Broken or disconnected CAN + conductor (open circuit)
  • Corroded or loose connector/pin at a module or splice
  • Short/open at a splice or wiring harness damage (chafing, rodent damage)
  • Failed CAN transceiver or control module
  • Missing or damaged termination resistor(s)
  • Water intrusion or contamination in connectors

Symptoms

  • Communication-related DTCs (U-codes) and MIL illumination
  • Loss or intermittent operation of modules on the LS-CAN network (e.g., body, door, mirror, HVAC, instrument cluster functions)
  • No or garbled data from one or more modules when scanned
  • Systems that rely on the LS-CAN remain inoperative or default to backup mode

What to check

  • Use a scan tool to confirm active U0021 and any additional communication codes; record freeze-frame and module list
  • Visually inspect LS-CAN wiring, connectors, splices and modules for damage, corrosion, or water entry
  • Check module power and ground circuits feeding the CAN transceivers (fuses, battery feed, chassis grounds)
  • Measure termination resistance between CAN+ and CAN− with ignition off (expected ≈ 60 ohms for a properly terminated two-terminator bus)
  • Perform continuity check of CAN+ conductor between modules and to splice points (with harness connectors disconnected where practical)
  • Use an oscilloscope or CAN bus monitor to observe CAN+ and CAN− waveforms while cranking/ignition on

Signal parameters

  • Clean, healthy CAN bus: recessive state — both lines near mid-supply (~2.5 V); dominant state — CAN_H rises (~3.5 V) and CAN_L falls (~1.5 V), producing ~2 V differential
  • Termination resistance (two 120 Ω in parallel) measured across CAN+ and CAN− with key off: ≈ 60 Ω
  • Open on CAN+ typically shows no toggling on CAN+ line and an idle voltage pulled toward module input or open-circuit voltage
  • Oscilloscope shows valid CAN frames (differential pulses) when network is healthy; an open conductor will remove expected differential transitions

Diagnostic algorithm

  1. Retrieve and record all active/pending codes and freeze-frame data. Note which modules are not responding on the scanner.
  2. Perform a visual inspection of LS-CAN harness routing, connectors, and modules for obvious damage or corrosion. Repair any obvious faults.
  3. Verify power and grounds to modules that interface to the LS-CAN (check fuses, fused links and chassis grounds). Repair as needed.
  4. With ignition off, measure resistance between CAN+ and CAN− at an accessible connector to verify termination (~60 Ω).
  5. Check continuity of the CAN+ conductor from the failing module connector back through splices to other network nodes. Repair any opens or high-resistance connections.
  6. With ignition on, use a multimeter/oscilloscope to monitor CAN+ and CAN− signals while comparing to a known-good vehicle or known-good segment. Look for absence of toggling on CAN+ (open) or abnormal voltages.
  7. Isolate the fault by disconnecting modules one at a time (where safe and practical) to see when the bus behavior or codes change—this can identify the segment or module causing the open.
  8. If a module is suspected, inspect and repair the connector and wiring; if harness and connectors are good and the bus is still open, replace or repair the defective module or its CAN transceiver per manufacturer instructions.
  9. After repairs, clear codes and verify network communications and affected systems return to normal during a test drive.
  10. Safety note: take usual precautions when probing live circuits. If performing soldering or splicing, disconnect battery where required by vehicle manufacturer procedures.

Likely causes

  • Damaged wire or pin at a harness connector where the LS-CAN branch connects
  • Corroded connector or poor pin-to-socket contact at a network module
  • Open splice or broken conductor inside loom
  • Failed module transceiver on one end of the LS-CAN segment
  • Removed or burnt termination resistor causing loss of network reference

Fault status

⚠️ Status
Low Speed CAN bus positive conductor open — communication lost or intermittent on LS-CAN network. Inspect wiring, connectors, terminations, and module transceivers.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

Similar codes

U0100 U0073 U0155 U0121 U0115 U0105
856

Browse 856 MERCEDES-BENZ manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

MERCEDES-BENZ

Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email

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